Those who drive vehicles on ice, snow or loose gravel frequently have trouble controlling the spin of the vehicle's driven wheels. Such surfaces have a relatively low coefficient of friction which substantially reduces the traction between the driven wheels and the road surface. As a result, even a small amount of torque applied to the driven wheels tends to cause them to spin. If the spin becomes severe, the vehicle can be difficult to control.
As far as the applicants are aware, there is no known technique for accurately controlling the wheel spin of a moving vehicle by varying engine torque in proportion to an estimated traction characteristic of the surface under the vehicle. A recent paper which refers to limited use of a surface adhesion coefficient in order to improved traction control is entitled "Development of New Control Methods to Improve Response To Throttle Type Traction Control System," by Hiroshi Igata et al., reprinted from Transmission and Driveline Symposium: Components, Gears and CAE, (SP-905), bearing the legend "International Congress & Exposition, Detroit, Mich., Feb. 24-28, 1992," available as SAE Technical Paper Series No. 920608. Page 4 of the paper states that the surface adhesion coefficient can be calculated from the acceleration of the nondriven wheels of a vehicle. The paper also states that if the adhesion coefficient is sufficiently high, throttle feedback gain can be increased without the occurrence of throttle hunting, and therefore, feedback gain should be determined according to the calculated adhesion coefficient. According to Igata et al., the adhesion coefficient merely serves to determine whether such feedback can be increased. Page 5 of the paper suggests that the calculated adhesion coefficient should not be revised to a lower value unless wheel slip exceeds a predetermined level. Page 3 of the paper indicates that an initial value of throttle feedback should be a function of adhesion coefficient and engine speed.
Contrary to the limited use of adhesion coefficient suggested by Igata et al., the applicants have discovered that improved wheel spin control can be achieved by varying engine torque in a manner which may be dependent or independent of the value of a traction characteristic of a surface under the vehicle, such as coefficient of friction, depending on the value of the characteristic.